This application is a National Stage entry of and claims priority to International Patent Application No. PCT/US2012/053093, filed on Aug. 30, 2012.
The present invention relates generally to downhole power generation systems and methods and, more particularly, to downhole power generation systems and methods that utilize a variable-output generator to generate electrical power.
Modern hydrocarbon drilling and production operations can often require that electrical power be supplied to equipment in the downhole environment. For example, electrical power is required downhole for a number of applications, including well logging and telemetry. Well logging of the borehole often includes the use of active sensors that require power to obtain information about the downhole environment. Such information will typically include the various characteristics and parameters of the earth formations traversed by the borehole, data relating to the size and configuration of the borehole itself, pressures and temperatures of ambient downhole fluids, and other downhole parameters. Telemetry commonly utilizes electrical power to relay data acquired from various logging sensors in the downhole environment to the surface.
Supply and generation of power downhole can be problematic for a number of reasons. Storage of power in certain regions of the wellbore can be problematic due to high temperatures and other harsh conditions that conventional batteries and capacitors cannot withstand. Generation of power downhole can be accomplished by using the circulating drilling mud to operate a generator or turbine located downhole. At least one problem with this approach is that mud flow rates can vary widely and the mud flow may need to be increased, for example when the drill bit enters a new type of subterranean strata. As the flow rate becomes excessively high, significant issues can be encountered. First, at high flow rates, the increased rotational rate produces high torques within the generator that can lead to its mechanical failure. Second, at high flow rates, more power can be generated than is necessary for the intended application. This excess power generation can lead to heat production, which can be detrimental to electronic components in the drill string. Additionally, due to the potential extreme flow rates that the circulating mud may attain, any such generator or turbine is typically oversized relative to the nominal requirement for power. The increased size of the generator or turbine leads to lower than optimal efficiency and increased cost.
A conventional approach to generating a constant amount of power when the mud flow must be increased is to pull the downhole generator and reconfigure the turbine to operate at the higher mud flow rate. Pulling, modifying, and re-inserting the downhole power generator requires a significant amount of time, thereby incurring an operational cost and delaying the drilling operation. There is currently no adequate resolution for this issue in regions of the wellbore in which power currently cannot be satisfactorily stored or delivered.